As pointed out by [1]}, in early 2002 it was easy to align
light curves obtained with different instruments, while later, due to the extreme colours of V838 Mon,
no comparison stars with matching colours have been available, and hence matching has not been trivial.
The offsets between published photometric datasets are also discussed in GOR20.
As an example, we plot in Fig. REF  our TO data from 2002
(in blue) together with literature values
(in black). A reasonably good match can be seen between the datasets.
In Fig. REF  we plot our \(BR\)  magnitudes
taken with NOT starting from 2009 together with the \(BR\)  data of GOR20
collected during the same period.http://www.vgoranskij.net/v838mon.ne3
A straight line is fitted through each dataset.
As can be seen, the fitted lines in the \(B\)  filter can be considered parallel,
implying a constant offset of about 0.4 mag
between our NOT and GOR20 standard magnitudes. In the \(R\)  band the fitted lines are
not parallel, probably due to the above-mentioned calibration difficulties.
The issue of offsets changing over time was also pointed out by GOR20.
